1. Field of the Invention
This invention relates generally to offshore floating vessels and platform structures and, more particularly, to a semi-submersible floating platform having a semi-submersible hull structure, with one or more passive damper weight members suspended below the hull by flexible connection members at variable distances to provide hydrodynamic mass, damping, and in-water weight to the platform during operation.
2. Background Art
Haselton, U.S. Pat. No. 3,986,471 discloses apparatus for damping vertical movement of a semi-submersible vessel having a small water plan area, whose buoyancy is provided substantially completely by submerged pontoon means and which includes at least one submerged damper plate supported deep beneath the vessel by flexible tensioned support elements, such as chains or cables and which has valves or similar flow controllers therein for providing substantially greater resistance to the upward movement of the plate than the downward movement. The area of the damper plate is several times larger than the water plane area of the vessel and provides low resistance to downward movement and higher resistance to upward movement.
Xu et. al, U.S. Pat. No. 6,652,192 discloses an apparatus for a heave suppressed floating offshore drilling and production platform having vertical columns, lateral trusses connecting adjacent columns, a deep submerged horizontal plate supported from the bottom of the columns by vertical truss legs and a topside deck supported by the columns. During the launch of the platform and towing in relatively shallow water the truss legs are stowed in shafts within each column and the plate is carried just below the lower end of the columns. After the platform has been floated to the deepwater site, the truss legs are lowered from the column shafts to lower the plate to deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform.
Wetch, U.S. Pat. No. 6,666,624 discloses a floating platform comprising an uppermost buoyant and ballastable hull partially submerged in water without contacting the floor of the body of water and usually without being moored to the floor of the body of water. The bottom of the uppermost hull is attached to the top of a lower buoyant and ballastable hull after the lower hull has been completely submerged and anchored to the floor of the body of water with flexible mooring.
Srinivasan, U.S. Pat. No. 6,761,124 discloses column stabilized floating structures having a deck and a plurality of vertical buoyant caissons bridged together in distantly spaced relation by plurality of open frame horizontal truss pontoon members and a vertical truss column at the lower end. The buoyancy of the caissons is selectively adjusted by means of ballast control. Water is selectively pumped into and out of the keel tanks at the bottom of the truss structure to raise or lower the center of gravity of the entire mass of the floating structure relative to the center of buoyancy to compensate for different operational, environmental and survival conditions.
Merchant et al, U.S. Pat. Nos. 7,156,040 and 7,219,615 disclose a semi-submersible vessel with a pair of vertically spaced pontoons with varied buoyancy. The lower pontoon is retained in a close vertical proximity to the upper pontoon when the vessel is in transit. The lower pontoon is ballasted at the deployment site dropping the pontoon to a depth of about 32 m below the first pontoon baseline to improve stability and motion characteristics of the vessel. The lower pontoon supports a plurality of vertically movable retractable legs that move through leg guides carried on the exterior of stabilizing columns and the upper pontoon. The legs ensure that the second tier pontoon is retained in a parallel relationship to the upper pontoon.
Liden, U.S. Pat. No. 4,498,412; Frimm et al, U.S. Pat. No. 6,015,245; Huang et al, U.S. Pat. Nos. 6,447,208 and 6,505,023; Key et al, U.S. Pat. No. 6,701,861; and Wybro et al, U.S. Pat. No. 7,140,317 disclose offshore floating platforms and vessels having vertical support columns and ring pontoon structures, but do not disclose passive damper weight members suspended below the hull by flexible connection members. Moougin, U.S. Pat. No. 4,241,685 and Horton U.S. Pat. No. 6,817,309 discloses single-column or spar-type offshore floating structures having a cylindrical column or tower divided into damping chambers, but do not disclose passive damper weight members suspended below the column or tower by flexible connection members.